Recently, a projector, which projects images on various projection surfaces such as a screen, has been widely used. A lamp is generally used as a light source of the projector. The lamp has, however, a short life. A color reproduction range of the lamp is also limited. In addition, there are drawbacks of a large required area and inefficient light utilization of the lamp.
It is attempted that a laser source is used as the light source of the projector, in order to solve the aforementioned problems. The laser source has a longer life and a stronger directivity than the lamp. Accordingly, it is likely that the laser source efficiently utilizes light. The laser source is monochromatic to achieve a large color reproduction range. Therefore, the projector utilizing the laser source may display vivid images.
If a smaller laser source than a conventional lamp is utilized as a light source of a projector, various optical elements incorporated in the projector are also miniaturized because the laser source is a point light source. Accordingly, it is expected that it contributes to development of a novel portable projector if the laser source is applied to the light source of the projector. The projector utilizing the laser source is called a “laser projector” in the following descriptions.
The laser projector has a problem about speckle noise. The speckle noise results from high coherence of the laser beam. It is known that the speckle noise occurs in a diffracted field so that a pattern, which is caused in an optical system, appears on a display surface and in an image field so that viewer's eyes perceive resultant fine particulate noise from scattered laser beams on the display surface. The former speckle noise in the diffracted field may become problematic in an exposure device and an illumination optical system. Both speckle noises in the diffracted field and the image field become problematic in the laser projector.
If a projector is miniaturized, an optical system has to illuminate a spatial light modulator uniformly enough to modulate the laser beams and create image light, in addition to utilizing characteristics of the point light source, which is different from an optical system such as a conventional lamp or LED as the light source. The miniaturization of the projector also requires efficient light utilization to overcome resultant problems from heat generation.
If the laser source is used as the light source of the projector, light intensity has to be sufficiently decreased on the retina of a viewer in terms of safety reasons. Accordingly, the light intensity of the laser beams emitted from a projector lens has to be reduced or the light has to be prevented from condensing on the retina of the viewer.
There are various proposals for reduction in the speckle noise and enhanced safety of the laser projector (c.f., Patent Documents 1 and 2).
The laser projector disclosed in Patent Document 1 includes a laser source; a beam spreader; a beam shaper, which consists of two sets of fly's eye lenses, a condensing lens and a field lens; and a moving diffuser. The combination of the beam shaper with the moving diffuser results in uniform illumination to the spatial light modulator and decreased speckle noise.
The laser projector disclosed in Patent Document 1, however, requires a relatively large optical system (as large as the optical system used together with the lamp). Accordingly, such a laser projector is unsuitable for the miniaturization of the optical system. In addition, the disclosed laser projector achieves an insufficient level in terms of the reduction in the speckle noise and safety of the laser projector.
Patent Document 2 proposes that an optical power is decreased to a prescribed level if it is detected that a person comes into an area near a projection area in which the laser beam is projected. Patent Document 2 discloses technologies, which prevent intensive light from condensing on the human retina.
The projector of Patent Document 2, however, has a complicated structure. Therefore, the projector of Patent document 2 is likely to become a large size. Accordingly, an optical design which always prevents the intensive light from condensing on the human retina without reducing the optical power is desired as a basic safety measure.
It is proposed to utilize an element such as a diffraction element or HOE element, which is processed by a wavelength order, as a beam shaper of the laser source. The utilization of these elements as the beam shapers results in a limited usable range of wavelengths and incident angles. Accordingly, the utilization of such elements as the beam shapers is not directed to make light intensity uniform and precisely shape beams if there are several wavelengths and light sources. In addition, it becomes likely that the utilization of these elements as the beam shapers causes resultant spots of power concentration from 0-order light.
In the aforementioned proposals, the optical design is not sufficiently examined for reducing the speckle noise and achieving a high luminance and a high safety while maintaining a compact size of the laser projector.
Patent Document 1: JP 2003-98476 A
Patent Document 2: JP 2006-227083 A